Problem using invoice and wallpaper packages together

Question

For some reason that I don't understand, it seems that I cannot use the invoice package and the wallpaper package at the same time.

Here is my MWE:

\documentclass[a4paper,10pt]{article}
\usepackage[utf8x]{inputenc}
\usepackage{wallpaper}
\usepackage{invoice}

\begin{document}
Just a test
\end{document}

And this is the output I get after running pdflatex:

If I open the transcript file, there's something that seems to be related to the problem:

(/usr/share/texlive/texmf-dist/tex/latex/latexconfig/epstopdf-sys.cfg
File: epstopdf-sys.cfg 2010/07/13 v1.3 Configuration of (r)epstopdf for TeX Liv
e))
Overfull \hbox (15.31238pt too wide) in paragraph at lines 184--92
[]\OT1/cmr/m/n/10 [2007/09/09]    [2010/03/01]  [2010/12/02]  [2010/09/13]
 [2007/11/11]    [2007/09/09]            [2007/09/09]

I also tried loading the packages in a different order, like this:

\documentclass[a4paper,10pt]{article}
\usepackage[utf8x]{inputenc}
\usepackage{invoice}
\usepackage{wallpaper}


\begin{document}
Just a test
\end{document}

In this case pdflatex gives an error message, complaining about a missing \begin{document} in wallpaper.sty:

(/usr/share/texlive/texmf-dist/tex/latex/wallpaper/wallpaper.sty

! LaTeX Error: Missing \begin{document}.

See the LaTeX manual or LaTeX Companion for explanation. Type H for immediate help. ...

                                               l.24 \NeedsTeXFormat{LaTeX2e}[
                          1995/06/01]

If I use only one of the packages, the document gets rendered as expected.

So here are my two questions:

1. Why is this happenning?
2. How can I fix it?

Answer 1

Someone please contact the invoice author. He is loading fp via \input{fp} instead of \RequirePackage{fp}. If I change that line in invoice.sty then it seems to work just fine.

Answer 2

A minor work-around for this is to make \input{fp} conform to \RequirePackage{fp} when using \usepackage{invoice}. Here's one way of doing that (which uses pdfTeX's \pdfstrcmp):

\let\oldinput\input% Save \input
\renewcommand{\input}[1]{%
  \ifnum\pdfstrcmp{#1}{fp}=0
    \RequirePackage{fp}% Specifically when finding \input{fp}
  \else
    \oldinput{#1}% If not \input{fp}
  \fi
}
\usepackage{invoice}% http://ctan.org/pkg/invoice
\let\input\oldinput% Restore \input